Conventional methods for designing tires are based on empirical rules achieved by a repetition of numerical experiments using actual experimentation and computers. Therefore, the number of trials and tests required for development is extremely large, which increases development costs, and the development time period cannot be shortened easily.
For example, the shape of the crown portion of a tire is designed on the basis of several arcs in a cross-sectional configuration including a rotational axis of the tire. A value of an arc is determined from data obtained by preparing several molds and testing and evaluating tires prepared from the molds, or is determined by conducting many numerical experiments. Therefore, the development efficiency is not good.
Further, pattern design has many degrees of freedom. Therefore, after grooving a proposed basic pattern in a tire or after actually preparing a mold, a trial tire is made and tested on a vehicle and evaluated. Problems arising at the vehicle are overcome by finely modifying the proposed basic pattern, so as to complete a final pattern. Thus, pattern design is in a field requiring the most processes, as compared with the designing of tire shape and structure.
A pneumatic tire is generally formed with rib grooves in a circumferential direction of the tire and lug grooves in a radial direction of the tire, so as to prevent the hydroplaning phenomenon which occurs during vehicle running in rain, and so as to ensure braking performance and traction performance. A general pattern is a so-called block pattern which includes island shaped land portions surrounded by these rib grooves and lug grooves.
Such a block pattern requires running performances of the tire, in general, both a straight running performance and a cornering performance. The straight running performance requires a grip force in a circumferential direction of the tire, and a relatively hard rubber is suitable. On the other hand, the cornering performance requires a grip force in a widthwise direction of the tire, and a relatively soft rubber is suitable to increase the grip force during cornering. Due to the soft rubber, there is need to increase energy loss, which is antinomical.
For this reason, in order to obtain a plurality of antinomical performances, there has been proposed a so-called widthwisely divided tread in which the tread is divided in a widthwise direction, a soft tread rubber is used in regions in the vicinities of both ends of the tread which contribute greatly to cornering, and a hard tread rubber is used in a center region of the tread which contributes greatly to straight running (Japanese Patent Application Publication Nos. 58-50883 and 63-23925).
However, a tire provided with the widthwisely divided tread as in the conventional art is inferior in productivity, and has a problem that irregular wear or separation occurs at a boundary surface of the division.
Further, in many cases, a block shape of the ground- contacting surface of the tire tread portion is determined so as to prevent the hydroplaning phenomenon, or is determined by requirements in terms of braking performance, traction performance, and design which is fit for aesthetic appearance, and therefore, the degrees of freedom in design are largely limited.
In view of the above circumstances, it is an object of the present invention to provide a method for designing a pneumatic tire, in which when a single performance or a plurality of antinomical performances are to be obtained, the best mode of a tire is designed under a given constraint conditions, and in which the tire can be efficiently designed and developed.